This application is based on and claims the priorities under 35 U.S.C. xc2xa7119 of German Patent Application 198 60 271.5, filed on Dec. 24, 1998, and German Patent Application 199 55 145.6, filed on Nov. 17, 1999, the entire disclosures of which are incorporated herein by reference.
The invention relates to a method for carrying out anisotropic plasma-chemical dry etching of silicon nitride layers using an etching gas mixture particularly containing SF6 and CHF3.
Various techniques of plasma-chemical dry etching, such as reactive ion etching (RIE) for example, are well known and are typically used in the fabrication of semiconductor circuit elements. An advantage of such dry etching techniques in comparison to wet etching techniques is that structures having dimensions of less than 1 xcexcm can be produced using dry etching but not by using wet etching. This is necessary, for example, for fabricating integrated circuit elements with SiGe transistors.
U. S. Pat. No. 5,433,823 (Cain) discloses a method of carrying out so-called pad window etching using a gas mixture of SF6 and CHF3, among other gas mixtures. Especially, the mole ratio of CHF3 relative to SF6 is in the range from about 5/1 to about 20/1. The gas flow rates used in the process are a CHF3 flow rate of 180 sccm (standard cubic centimeters per minute) and an SF6 gas flow rate of 20 sccm. The disclosed conventional method is used for etching a passivating semiconductor layer sequence or stack including a layer of Si3N4 having a thickness of approximately 1 xcexcm and an underlying layer of SiO2 having a thickness of approximately 0.5 xcexcm. Both the Si3N4 and the SiO2 are etched relative to a metal layer of TiW arranged lying under the two semiconductor layers, so as to entirely remove both the Si3N4 and the SiO2 at a specified window location. The structural dimensions to be etched by the conventional method are in a range around 100 xcexcm.
Table 3 in column 9 of U.S. Pat. No. 5,433,823 shows the etching selectivities of the etching of the silicon nitride layer relative to the resist material, and of the silicon oxide layer relative to the resist material, when using a conventional SF6 etching gas in one case (the left data column of Table 3) and when using the disclosed mixture of SF6 and CHF3 etching gas in another case (the right data column of Table 3). While the patent points out that the etching selectivity of the silicon nitride relative to the resist and of the silicon oxide relative to the resist, respectively, is improved by using the gas mixture of SF6 and CHF3, a significant disadvantage is also apparent from the data shown in Table 3.
Namely, while the SF6 etching gas achieves a selectivity of 1.6 when considering the silicon nitride relative to the silicon oxide (1.6/1.0=1.6), the disclosed mixture of SF6 and CHF3 achieves a selectivity of only 0.83 for etching silicon nitride relative to silicon oxide (3.3/4.0=0.825). Thus, U.S. Pat. No. 5,433,823 suggests that the addition of CHF3 to SF6 in an etching gas mixture reduces the etching selectivity of silicon nitride relative to silicon oxide from 1.6 to 0.83. While that may not be significant in the context of the patent, wherein a silicon nitride layer and a silicon oxide layer are both to be etched selectively relative to a resist layer and a metal underlayer, it is a disadvantage in any application in which a silicon nitride layer is to be etched selectively relative to a silicon oxide layer.
In various conventionally known etching methods, CF4, CHF3 or other fluorine-containing gases or gas mixtures are typically used in combination with O2 as disclosed in International Patent Publication WO 96/16437. Published European Patent Application EP 0,706,206 similarly discloses an etching process in which a mixture of CF4+O2 is utilized. In all dry etching processes using O2 as a component of the etching gas mixture, there arises the substantial disadvantage that such processes, i.e. such gas mixtures, cannot be used in the reactor chambers of dry etching systems that use oxidizing or oxidizable electrode materials such as silicon or carbon, because such electrode materials would be attacked and corroded due to the effects of the oxygen component of the gas.
Another type of process is disclosed in German Patent 37 14 144, for example. This German Patent suggests to use a fluorine-containing gas together with chlorine or bromine as a gas mixture. Since these gases, or gas components, are corrosive and toxic, they are not suitable for use in all reactors.
German Patent Laying-Open Document 42 32 475 discloses a plasma-chemical dry etching process for selectively etching silicon nitride layers relative to silicon oxide layers. The etching gas or etching gases used in the disclosed process contain compounds in which respectively one fluorine atom and at least one atom selected from the group consisting of chlorine, bromine and iodine are chemically bonded onto a hydrocarbon framework in the molecular structure. Due to the corrosiveness and toxicity of the gas or its components, such an etching gas is also not suitable or acceptable in all applications.
International Patent Publication WO 96/16433 discloses an anisotropic and selective dry etching process for silicon nitride over thin silicon oxide layers, in which only Cl2 is used as the etching gas. This leads to very low etching rates, which are not practically applicable for etching relatively thick layers, or any layer other than the very thinnest layers.
Published European Patent Application EP 0,516,053 discloses a process in which a mixture of S2F2, SF2, SF4 or S2F10 with an inert gas is used as the etching gas, especially in order to etch SiO2 selectively relative to Si3N4. The reference discloses that the sulfur that is freed or released during the etching process is redeposited as a passivating layer on the Si3N4 surfaces and thereby hinders the etching of the Si3N4. Through this mechanism, the SiO2 may be selectively and preferentially etched in comparison to the Si3N4.
In the context of the above discussed conventional etching processes, and in addition to the above mentioned disadvantages of the prior art when using oxygen and halides especially containing Cl2 and Br, studies and experiments conducted by the present inventors have shown that all previously known processes are unable to achieve an adequate or satisfactory control of the edge slope angle of the etched edge of the silicon nitride material. Namely, the resulting slope angle of an edge of an opening or etched-away area of the silicon nitride layer, which is exposed by a corresponding opening in a resist layer or mask, is variable and not readily controllable using the prior art processes.
With the exception of the above mentioned pad window etching process disclosed in U.S. Pat. No. 5,433,823, the other prior art processes described in the literature are used for etching Si3N4 layers having a thickness of less than about 150 nm. With such thin layers, it is not necessary or critical to achieve a high degree of control of the edge slope angle. If the layer thickness is greater than 150 nm, however, then the resulting edge slope angle does become important. Thus, if the layer thickness of the Si3N4 material to be etched is greater than 150 nm, or if the edge slope angle serves an important function for the semiconductor component being fabricated, then the prior art processes are not adequate for achieving the required degree of control of the etched edge slope angle.
In view of the above, it is an object of the invention to provide a method of the above described general type for the plasma-chemical dry etching of silicon nitride layers selectively relative to silicon oxide layers, wherein the edge slope angle of an etched edge of a silicon nitride layer can be precisely set to a required value, while achieving a high etch rate. The invention also aims to avoid the use of corrosive and/or toxic gas components so that the consequent disadvantages can be avoided. Furthermore, the invention aims to avoid or overcome the disadvantages of the prior art, and to achieve additional advantages, as are apparent from the present specification.
The above objects have been achieved according to the invention in a method of carrying out anisotropic plasma-chemical dry etching of a layered arrangement including a second or upper semiconductor layer comprising Si3N4 arranged on a first or lower semiconductor layer comprising SiO2, using an etching gas mixture that contains SF6, CHF3 and a further non-oxidizing gas. A resist layer or mask layer is arranged on top of the upper semiconductor layer, and the etching is carried out through a patterned opening of the resist layer, whereby the etched edge of the semiconductor layer comprising Si3N4 is formed at a certain edge slope angle xcex1. Preferably according to the invention, the relative proportions of the gas components in the etching gas mixture and/or other process parameters are adjusted in order to adjust the edge slope angle xcex1 of the resulting etched edge and to adjust the selectivity of the etching of the upper semiconductor layer comprising Si3N4 relative to the lower semiconductor layer comprising SiO2.
Studies and experiments carried out by the present inventors have shown that the inventive mixture of CHF3 and SF6 together with a non-oxidizing gas unexpectedly achieves very good and improved process characteristics and etching results. Particularly, it has been determined that the inventive gas mixture achieves an especially good etching selectivity of silicon nitride relative to silicon oxide while achieving a high etching rate and while simultaneously allowing an adjustment or selection of the resulting edge slope angle of the silicon nitride layer that is to be etched. It appears that the two gases CHF3 and SF6 do not simply act as suppliers or donors of the etching fluorine, but instead it is particularly the mixture or combination of the components of these two gases containing hydrocarbon and sulfur, further in combination with a non-oxidizing gas, that leads to the unexpected new and improved characteristics and results. In this context, the non-oxidizing gas does not simply function as a carrier gas, but instead the non-oxidizing gas represents an active etching component through a reciprocal or mutual interaction with the other gases of the mixture.
An advantage of the presently achieved high silicon nitride etching rates is that relatively thick silicon nitride layers, for example having a layer thickness greater than 200 nm, can be etched with a relatively low process time and thus a high process throughput and economic efficiency, while also carrying out the etching with a defined edge slope angle. A further advantage of the inventive method or process is that a vertical edge slope angle of the silicon nitride layer that is to be etched can be selected or adjusted as needed. In a case in which a vertical etched edge is formed, such an edge can further be provided with a silicon oxide spacer in subsequent process steps, for example.
In the event that a noble inert gas, such as argon for example, is used in the inventive method as the non-oxidizing gas, it is possible to achieve an etching selectivity of silicon nitride relative to silicon oxide of greater than 2, already by adding only a small amount of CHF3 to the SF6 in the gas mixture. The use of argon relative to helium, for example, achieves the additional advantage that a particularly good uniformity of the silicon nitride etch rate over the silicon nitride wafer surface can be achieved, presumably due to the relatively high atomic weight of argon relative to helium. As a result, it is possible to completely and reliably remove all of the silicon nitride layer using a relatively short over-etching time, for example of only 15% or less, or preferably even less than 10% (e.g. 9 or 9.5%) of the total etching time.
By appropriately selecting or adjusting the process parameters of the dry etching process, and particularly the flow rates of the gas mixture, the pressure in the etching reaction chamber of the dry etching system, the plasma generating power of the RF generator, and the electrode gap or spacing, it is possible to select the desired or required resultant edge slope angle of the etched edge within an edge slope angle range of more than 20xc2x0. Since the inventive gas mixture contains a non-oxidizing gas instead of the conventionally known use of oxygen and halides added to SF6 and CHF3, the inventive method can be used in reactor chambers equipped with oxidizing or oxidizable electrodes. Particularly, the gas mixture does not contain oxygen, chlorine, bromine, iodine or halides added to the above mentioned constituents.